The present invention relates to a method of producing monomer vinyl chloride (VCM), wherein a balance is maintained between the hydrogen chloride (HCl) produced and consumed in the various reactions.
The process for the production of VCM with a adequate HCl balance, hereinafter referred to as xe2x80x9cbalanced VCM processxe2x80x9d, comprises the following process steps:
direct chlorination in which part of the required intermediate product 1,2-dichloroethane is produced from ethylene (C2H4) and chlorine (Cl2);
oxychlorination in which the other part of the required intermediate product 1,2 dichloroethane is produced from ethylene, hydrogen chloride (HCl) and oxygen (O2);
EDC purification in which the by-products generated in the oxychlorination and EDC pyrolysis sections are removed from both partstreams of the intermediate EDC product and from the EDC recycled from the fractionation section in order to obtain a so-called crackable EDC suitable for use in the pyrolysis section;
EDC pyrolysis in which the crackable EDC is cracked thermally, thus basically yielding VCM, HCl and non-reacted EDC in the cracked gas plus some additional co-products; and
fractionation in which the desired pure EDC product is separated from the cracked gas while simultaneously recovering useful substances (HCl and non-reacted EDC) contained in the cracked gas, these substances being recycled in the balanced VCM process.
In a commercial-scale plant operating by the balanced VCM process, the oxychlorination of ethylene proceeds preferably in a catalytically acting fluidised bed. Publication (McPherson, R. W. et al., xe2x80x9cVinyl Chloride Monomer . . . What You Should Knowxe2x80x9d, Hydrocarbon Processing, 58th edition, March 1979, page 75-88) explains that the reaction temperature may not exceed 325xc2x0 C., because the amount of by-products obtained could otherwise become unreasonably high.
The oxychlorination of ethylene is a distinctively exothermal reaction. Because of the limited temperature range of the reaction, it is imperative that the generated heat be dissipated directly. Hence, the fluidised bed is normally brought into contact with suitable surfaces, the purpose of which is to effectively extract heat from the fluidised bed.
The heat-extracting surfaces and the other surface in contact with the material of the fluidised bed are subject to wear as a result of the fact that the fluidised bed material contains corundum. These surfaces are also likely to corrode, because one of the gaseous reaction products in the fluidised bed is hydrogen chloride that forms on the surfaces.
It is known from GB-PS 1 100 439 (Harping et al. ) that a copper catalyst on a corundum (Al2O3) carrier is used for the oxychlorination of ethylene. The bulk catalytic material used in the fluidised bed has a copper content of about 10% (wt).
NL-PS 65/06985 states that the catalytically acting copper impregnate is deposited on spherical particles of corundum, thus permitting a copper content of about 10% (wt) to be adjusted. The size of the corundum particles is in the order of 20 xcexcm to 200 xcexcm.
It can further be seen from U.S. Pat. No. 3,488,398 (Harping et al.) that the catalytically acting copper impregnate used is in the form of copper halides, preferably copper chloride. It is stated that a copper content of 3% (wt) to 12% (wt) is practicable, but that copper contents of more than 12% (wt) can also be used, although this is not expedient, because the reaction yield cannot be increased by this method and the catalytically acting fluidised bed tends to cause incrustations in the oxychlorination reactor to an increasing extent.
Since the introduction of the balanced VCM process, a number of proposals have therefore been made for reducing the production of detrimental by-products in the oxychlorination reactor while simultaneously permitting the reactor to operate reliably without any fouling by incrustations and reducing the inherent abrasion and/or corrosion to a reasonable level.
Published patent application WO 96/26 003 (Krumbock) describes an oxychlorination reactor in which a reactant gas stream is fed into the fluidised bed in counter-current to the fluidising gas stream flowing in an upward direction. The primary purpose of this arrangement is to reduce or eliminate abrasion.
All operators of VCM plants using the balanced VCM process require that these plants be such that they are able to produce VCM during prolonged periods without any standstills caused by the particular operating conditions or malfunctions. The requirement for oxychlorination reactors is that a stable temperature range be achieved and maintained in the fluidised bed over prolonged periods even when the grain size spectrum of the fluidising bulk material, which consists of spherical particles of corundum, shifts to lower values due to abrasion.
The average grain size of fresh fluidising bulk material is in the order of 50 xcexcm to 60 xcexcm. Due to abrasion, the grain size gradually decreases to particle sizes of 30 xcexcm to 40 xcexcm.
The aim of the present invention is, therefore, to define a balanced process for the production of VCM, comprising the steps of:
(a) feeding gaseous reactants and an inert carrier gas to an adiabatic reaction zone in an oxichlorination section;
(b) transferring the gases and the portion of the gaseous reaction products formed in the adiabatic reaction zone to a cooled reaction zone arranged above the adiabatic reaction zone and converting the remainder of the reactant, wherein both reaction zones are associated with an integral bed which is fluidised by the gases and in which the catalytic reaction takes place; and
(c) withdrawing the gaseous reaction product from the fluidised bed in an upward direction,
wherein the cooled reaction zone contain a vertically coaxial hollow cooling rod bundle which forms a passage, the hollow cooling rods of which being almost equidistantly spaced,
wherein the part of the fluidised bed that is arranged in the cooled reaction zone is exposed to the heat-dissipating surfaces of the hollow cooling rods and the fluidising bulk material, the gases being conducted through the passages between the hollow cooling rods in a manner which permits dissipation of their heat, and
wherein the ratio of the equivalent diameter representing the passage cross-sectional area to the mean gas bubble pocket diameter of the fluidised bed is 1 to 6 inclusive.